A method and device of the kind referred to above are described in European Patent 0 084 037. In the known method and device, the characteristic field determines the desired charging pressure taking into account the output power, the composition of the exhaust gas, soot content of the exhaust, etc. As a result, supercharged diesel engines employing such method and device are supplied with too much air for combustion during most of the time of operation, for example when travelling at a comfortable cruising speed. The excess air is simply compressed and then expelled. The result is poor efficiency and high fuel consumption.